The object of froth flotation is to separate grains of different minerals from each other. The process is based on various properties of the free mineral surfaces. These properties can be affected by suitable treatment, but since normally all grains undergo the same steps of preparation, the separation is always based on the inherent physical and chemical characteristics of the mineral surfaces. In the traditional constructing of flotation circuits these characteristics are not analysed nor consistently utilized. Contrary to this, the new method to be described aims to determine certain parameters and variables describing the fundamental characteristics of different minerals and to establish the design, construction and control of flotation circuits on such information.
As for the factors determining the flotation behaviour of a mineral, the flotation rate coefficient k is commonly employed to describe the speed of flotation. Its value is different for different minerals; in this connection the individual mineral is understood to be specific with regard not only to mineral composition but also to grain size, among other things. Grains which are equivalent also in this respect will hereinbelow be considered to belong to the same grain type. Even grains of the same type may have different coefficients, if e.g. they have been subjected to treatment by chemical conditioning of different degrees.
In the article "A study of dynamic control properties of industrial flotation processes" by A. Niemi in Acta Polyt. Scand. Chem., No. 48 2nd ed., Helsinki 1972 the flotation phenomenon has been shown to be a first-order irreversible kinetic process. ##EQU1## C = concentration of the grain type in the flotation suspension k = flotation rate coefficient
The significance of the flotation rate coefficient in view of the operation of the flotation circuit is presently understood, and by means of this coefficient the operation of circuits may be analysed both in the steady state, as shown by T. Imaizumi and T. Inoue in Proc. VI Int. Min. Proc. Congr., Cannes 1963; Pergamon 1965, p. 581, and in a dynamically changing state (Niemi, Loc. Cit.). The dependence of the coefficient on the basic properties of the mineral surface and on parameters characterizing the conditioning has, however, not been clarified in a manner which would enable it to be utilized consistently in such constructing work which covers both the process of flotation and the preceding conditioning steps.
The value of the rate coefficient of each grain type is best determined under known standard conditions, e.g. in an elected flotation cell of laboratory type with constant speed of rotation of the impeller, constant air flow, etc. For determination of the coefficient the floating material is gathered and its amount is determined at predetermined intervals (Imaizumi & Inoue, Loc. Cit.).
The degree of conditioning of a grain can be expressed by the relative adsorption .GAMMA. of the collecting agent, which in its turn is equivalent to the amount of adsorbed collector in proportion to the amount of chemical corresponding to full monomolecular coating. The dependence of the coefficient K on .GAMMA. has not been determined. However, when a system comprising conditioning and flotation is constructed, the determination and understanding of this relationship is of central importance.
The adsorption of the collecting agent on the surface of a mineral grain is a kinetic process which can be described by the following differential equation (Niemi, Loc. Cit.): ##EQU2## X = adsorption rate coefficient C.sub.x = concentration of collecting agent in the liquid phase
This equation is valid for each mineral type. In view of industrial conditioning, it is important to known the value of the adsorption rate coefficient X for each type of grain.
The adsorption rate coefficient X is a parameter characterizing the inherent surface-physics properties of the grain; and the variable .GAMMA., and furthermore k, depends essentially on its value.
However, the value also depends on other factors, which are: the pH of the surrounding liquid phase, and various chemicals dissolved therein. The process of adsorption is also intentionally acted upon in practice in order to produce flotation of the desired kind. In such case, X has to be determined under the desired chemical conditions. It is often expedient to determine its value by a series of experiments as a function of the concentrations of various chemicals and of the pH value.
In a full scale industrial process the conditioners and flotation cells are of the continuous flow type, differing from laboratory apparatus. The construction of an industrial flotation plant is usually based on relationships which have been determined in the laboratory or on a pilot plant scale, and which usually are presented in the form of families of curves or equivalent. The parameters mentioned above and their interrelationships or their dependence on different variables determined in the laboratory are then not utilized.